The localization of 125I insulin in the blood vessels of the calf will be determined by quantitative autoradiography. Highly labelled 125I insulin will be added to blood perfused through the central retinal and choroidal vessels. The specificity of 125I insulin binding will be assessed by simultaneous perfusion with excess "cold" insulin and insulin analogues. The experiments will complement previous studies on insulin binding by an isolated calf retinal blood vessel preparation. The possible "intermalization" of insulin found in biochemical studies will be tested in calf eyes perfused and studied by EM autoradiography.